Pulsar Surveys

Pulsars have tremendous untapped potential to probe the behaviour of matter, energy, space and time under extraordinarily diverse conditions. Even though pulsars are frequently getting discovered with ongoing surveys at major telescopes over the world, the presently known pulsar population is only 1\% of that predicted by stellar synthesis models. Modeling by Faucher-Giguere et al. (2006) indicates that the Galaxy contains about 100,000 pulsars, while only a few thousand are known today! This implies that the vast majority of pulsars is waiting to be discovered.

Studies of pulsars yield a better understanding of a variety of physics problems, from acceleration of particles in ultra strong magnetic fields (primarily via the study of the emission properties of normal pulsars, having spin period > 30 ms) to probes of ultra-dense matter (mostly via studying the timing properties of millisecond pulsars, having spin period <30 ms, that are very stable rotators). The fractional rotational stability of millisecond pulsars (MSPs), one part in a million billion, is comparable to that of atomic clocks (Lorimer et al. 2004). Such rotational stability, compactness second only to black holes, and their presence in binary systems, make MSPs ideal laboratories to test the physics of gravity and as detectors for long-wavelength gravitational waves. Moreover, MSP evolutionary processes can be tracked through individual interesting discoveries of MSPs in special evolutionary phases (e.g. Roberts et al. 2011, Archibald et al. 2009; Roy et al. 2015). In addition to the regular radio emission from pulsars, millisecond transient bursts, called Fast Radio Bursts (FRBs; Lorimer et al. 2007, Thornton et al. 2013) are observed at the locations of dynamic events, making them useful probes for extreme matter states.

The high sensitivity of the GMRT at low frequencies (<~ 800 MHz) makes it an outstanding telescope for pulsar surveys; such surveys are hence an important research area at NCRA. With the aid of reduced quantised noise and high time resolution supported by the flexible GMRT Software Backend (GSB; Roy et al. 2010), the GMRT search sensitivity for MSPs improved significantly, resulting in the discovery of 8 MSPs from Fermi-directed searches (Bhattacharyya et al. 2013, Roy et al. 2015).

The outstanding GMRT potential for low-frequency pulsar surveys has been recently emphasized by the GMRT High Resolution Southern Sky (GHRSS) survey, a low-frequency survey for pulsars and transients away from the Milky Way's plane. The GHRSS survey covers Galactic latitudes |b|>5 degrees, scanning the southern sky, with declination -40 degrees to -54 degrees. This declination coverage is complementary to the coverage of other ongoing low-frequency sky surveys around the world. The first phase of the GHRSS survey was carried out using the narrow bandwidths of the GMRT Software Backend, at 322 MHz, and has already resulted in the discovery of 13 new pulsars (Bhattacharyya et al. 2016). The second phase, using the GMRT Wideband Backend and the 250-500 MHz receivers of the upgraded GMRT is now under way.

Low-mass X-ray binaries (LMXB's) and radio millisecond pulsars (MSP's) are linked through stellar and binary evolution, where MSP's are the end products of an episode of accretion of matter and angular momentum from the binary companion during the LMXB state. Over the last decade, the discovery of three transitional millisecond pulsars (tMSP's) has allowed a detailed study of the recycling process. Recent studies of PSR J1824−2452I and PSR J1023+0038 have observationally demonstrated the LMXB – MSP evolutionary link. These systems show direct evidence of back-and-forth state switching between radio MSP and accreting X-ray millisecond pulsar regimes and opened a new avenue of research in pulsar astrophysics. The third such tMSP system, J1227-4853, was discovered by us using the GMRT. PSR J1227-4853 is a 1.69 millisecond pulsar at a dispersion measure of 43.4 pc/cm^3. It transited into the active radio-MSP phase associated with a sudden drop of its X-ray and optical luminosity in 2012 December. Extreme orbital perturbations as well as the signature of proper motion are revealed from our GMRT timing campaign. This pulsar, an ``eclipsing redback'', is the only transitioning system currently in an active rotation-powered state. Simultaneous imaging and timing observations with the GMRT were used to directly show that eclipses are caused by absorption rather than dispersion smearing or scattering. A long-term timing study of PSR J1227-4853 is currently under way, which will help to determine whether these transitional systems will eventually be canonical radio MSP's or whether they form a new sub-class of MSP's that continue to transition between the two states. Also, such studies will result in better understanding of the spin evolution of the systems and the dynamics of accretion during the accretion-powered, propeller stage and the rotation-powered stage. The figure shows the pulsar search output for PSR J1227-4853 showing rapid evolution of period and period-derivative in a compact binary system.

The discovery of millisecond pulsars (MSP's) and their precise localisation using existing methods is hindered by their intrinsic fainter nature. This leads to significant delays between the discovery of MSP's and their further identification using conventional imaging methods. Motivated by the need for rapid localization of newly-discovered faint MSP's, we have developed a coherently dedispersed gating correlator for the GMRT. This gating correlator accounts for the orbital motions of MSP's in binary systems, while folding the visibilities with a best-fit topocentric rotational model derived from a periodicity search using simultaneously generated beamformer output. With this technique, the signal-to-noise ratio of the detection of an MSP in the image domain can be dramatically improved (by a factor of as much as 5). We have also incorporated a superior approach of dispersion correction, called coherent dedispersion, in our imaging technique to reconstruct the intrinsic pulse shape of such MSP's. We could unambiguously localize newly discovered Fermi MSP's in the on–off gated image plane with an accuracy of ±1”. Immediate knowledge of such a precise position enables the use of sensitive coherent beams of array telescopes for follow-up timing observations, which substantially reduces the use of telescope time (by a factor of 20 for the GMRT!). In addition, a precise a priori astrometric position reduces the effect of large covariance in timing fit, which in turn accelerates the convergence to an initial timing model. Moreover, such accurate positions allow for rapid identification of pulsar counterparts in optical and X-ray wavelengths.
Figure caption: On–off gated images for newly discovered Fermi MSP's. All the MSP's are marked in the respective 10’ × 10’ facet images.

Bhattacharyya et al. used the GMRT to perform deep observations to search for radio pulsations in the directions of unidentified Fermi Large Area Telescope (LAT) gamma-ray sources, resulting in the discovery of a new milli-second pulsar (MSP), PSR J1544+4947, an eclipsing MSP in a special evolutionary state. PSR J1544+4937 is a 2.16 ms pulsar in a 2.9-hour compact circular orbit with a very low-mass companion star (mass > 0.017 solar masses). At 322 MHz, the pulsar is found to be eclipsing for 13% of its orbit, whereas at 607 MHz the pulsar is detected throughout the low-frequency eclipse phase. Variations in the eclipse ingress phase are observed, indicating a clumpy and variable eclipsing medium. Moreover, additional short-duration absorption events are observed around the eclipse boundaries. The authors used the radio timing solutions to detect gamma-ray pulsation from the pulsar, confirming it as the source powering the gamma-ray emission.
The figure shows the frequency-dependent eclipsing detected with the GMRT in PSR J1544+4937. The pulsar radiation is seen to be eclipsed by the companion star at 322 MHz, but not at 607 MHz. The figure plots the variation of the timing residuals and the electron column density around the eclipse phase (which is indicated by the shaded region) at 322 MHz (top) and 607 MHz (bottom).

Bhattacharyya et al. used the outstanding GMRT potential for low-frequency pulsar surveys in the GMRT High Resolution Southern Sky (GHRSS) survey, a low-frequency survey for pulsars and transients away from the Milky Way's plane. The GHRSS survey covers Galactic latitudes |b|>5 degrees, scanning the southern sky, with declination -40 degrees to -54 degrees. This declination coverage is complementary to the coverage of other ongoing low-frequency sky surveys around the world. The first phase of the GHRSS survey was carried out using the narrow bandwidths of the GMRT Software Backend, at 322 MHz, and has already resulted in the discovery of bunch of new pulsars with exciting properties. Bhattacharyya et al. discovered 13 pulsars in the GHRSS survey in a surveyed area of 1800 square degrees, i.e. 0.007 pulsars per square degree, which is one of the highest among pulsar surveys away from the Milky Way’s plane. GHRSS survey discoveries include a millisecond pulsar (in a ~10 hour orbit around a ~0.18 solar mass companion star), a pulsar for which gamma-ray pulsations have been discovered using the Fermi Large Area Telescope, and two mildly recycled pulsars. The second phase, using the GMRT Wideband Backend and the 250-500 MHz receivers of the upgraded GMRT is now under way.
The figure shows the 21 pulsars discovered by the GMRT between 2012−2017 from targeted and blind surveys. Fermi-directed discoveries are shown as green points; the blue shaded region indicates the sky coverage in Galactic coordinates of the GHRSS survey, while the pulsars discovered in this survey are shown as red points
Details: http://www.ncra.tifr.res.in/ncra/research/research-at-ncra-tifr/research-areas/pulsarSurveys/GHRSS